What? Yes, he can. Solrock is slower, and can't switch into any of Nidoking's attacks except CB Earthquake. It can't surive any of Nido's attacks, and it won't be threatening a switch. It won't be hitting anyone back with EQ after it gets hit with a Sucker Punch. If your relying on out-predicting it with Solrock, good luck. Plus, Mega Horn takes it out fast.
Solrock is no Nidoking counter, unless you switch into CB EQ, even then, it's no counter. Steelix helps wall alot more, and can take an EQ or two. Plus, it can take Nido out with his out with it's STAB EQ, since it survives.
Of course, Steelix needs some sort of backup.